Understanding Rice Sensitivity to Fluridone Herbicide for Pigweed

The Problem

Palmer pigweed, an aggressive and herbicide-resistant weed, is becoming a major threat to Arkansas rice production, especially in furrow-irrigated rice systems where flooding is not used to suppress weed growth. Fluridone, a water-activated residual herbicide, has emerged as an effective new weapon against Palmer pigweed in furrow-irrigated rice. While furrow-irrigated rice isn’t flooded, the fields commonly hold water on the bottom part of the field during irrigation events. The potential for rice injury from fluridone — particularly in cultivars sensitive to early applications — poses a challenge to growers trying to balance effective weed control with crop safety and yield.

The Work

A research team, led by Jason Norsworthy, Distinguished Professor and Elms Farming Chair of Weed Science, conducted a two-year field study to evaluate rice-cultivar tolerance to fluridone. They tested 12 rice cultivars, applying fluridone at labeled and double-label rates, before and after the rice plant’s three-leaf stage. All experiments were conducted on silt loam soil in a flooded rice setting but offered insights into the herbicide in furrow-irrigated rice systems. The team also examined tank mixing options for growers dealing with already-emerged pigweed. The lead author of the study was Maria Souza, a graduate research assistant under the advisement of Norsworthy.

The Results

The study showed that fluridone tolerance varied significantly by rice cultivar and timing of application. However, no yield penalty was observed in any cultivar when fluridone was applied at the label rate after the three-leaf stage. Applying fluridone before the three-leaf stage caused reductions in shoot density, chlorophyll content, groundcover and delayed heading in most cultivars. Visible injury varied across years due to environmental conditions. For example, in 2023, applying twice the label rate led to yield reductions in most of the cultivars tested. The researchers found that tank-mixing fluridone with Loyant was effective against 3- to 4-inch Palmer pigweed, while propanil mixtures were only effective on very small weeds under 1 inch.

The Value

This research provides cultivar-specific guidance for rice growers using fluridone to control Palmer pigweed. By identifying timing and rate thresholds for safe use, the findings offer guidance to help farmers maximize weed control while avoiding crop damage and yield losses.